Light dispersing film and method of manufacture

ABSTRACT

A beaded light dispersing film has a substrate layer, and an optically transparent layer, having a predetermined thickness, disposed over a side of the substrate layer. Optically transmitting beads are arranged to penetrate at least partially into the transparent layer to define clear apertures at interfaces between the beads and the transparent layer. The bead radius is greater than the predetermined thickness. An absorbing layer is disposed on the transparent layer, in interstices between the beads. A method of manufacturing the film includes disposing optically transparent beads partially in an optically transparent layer disposed over a transparent substrate layer. The optically transparent layer has a thickness less than half a diameter of a transparent bead. An absorbing layer overlies the optically transparent layer.

This is a continuation-in-part of application Ser. No. 09/050,489, filedon Apr. 3, 1998, which is incorporated herein by reference.

BACKGROUND

The present invention is directed generally to a light dispersing filmand a method of manufacture, and particularly to a beaded lightdispersing film.

Beaded light dispersing films are used with rear projection screens andmonitors for transmitting an image from one side of a screen to a vieweron the other side. Such films typically include a number of small beadsattached to a substrate film, and an opaque layer disposed between thebeads so that whatever light is not transmitted through a bead isabsorbed by the opaque layer. The opaque layer also absorbs ambientlight incident on the film from the viewer's side, thus reducing thebackground light detected by the viewer.

Dispersing films are characterized by the gain, resolution,transmission, ambient light rejection and contrast, which properties aredetermined by the structure and materials employed in its construction.The gain is a measure of the intensity of the light transmitted by thefilm as a function of angle measured from normal incidence, and isdetermined, at least in part, by the refractive index of the beads andthe surrounding material. The viewing angle of a particular film isdefined as that angle at which the intensity is half the intensity ofthe light transmitted on-axis. The resolution of the film is determined,at least in part, by the size of the beads. Ambient light rejection andcontrast are affected by absorption of the opaque layer.

The interdependence of the optical properties of the various componentsof the film limit the optimization of the film characteristics. Thereis, therefore, a need to overcome this interdependence so that new filmsmay be produced having superior characteristics of gain, resolution,efficiency, ambient light rejection and contrast.

SUMMARY OF THE INVENTION

Generally, the present invention relates to a light dispersing film andits method of manufacture. In one embodiment the film has an opticallytransparent layer and optically transmitting beads arranged to penetrateat least partially into a first surface of the transparent layer todefine clear apertures at interfaces between the beads and thetransparent layer. An absorbing layer is disposed on the transparentlayer, in interstices between the beads. The beads penetrate into thetransparent layer to a depth selected to substantially achieve maximumtransmission of light through the optically transmitting beads, whilemaintaining essentially optimum contrast for the maximum lighttransmission.

In another embodiment, the film includes a layer of opticallytransparent material having a support surface and a plurality of beadsof optically transmitting material having a selected shape andrefractive index and arranged in a single-layer array on the supportsurface, each bead at least partially disposed within the layer ofoptically transparent material to produce an interface therewith, theinterface defining a clear exit aperture. A layer of light absorbingmaterial having a selected thickness is affixed to the layer ofoptically transparent material, for controlling ambient light rejectionof the light filter and to reduce light transmission through intersticesformed by the plurality of beads. The beads penetrate into the layer ofoptically transparent material to a depth selected to substantiallymaximize transmission of light through the plurality of beads.

In another embodiment, the film includes optically transmitting beadsarranged to penetrate at least partially into a first surface of atransparent layer. The beads define clear apertures at interfacesbetween the beads and the transparent layer. An absorbing layer isdisposed on the transparent layer, in interstices between the beads, anda transparent cover layer is disposed over the absorbing layer and theoptically transmitting beads.

A method of manufacturing the film includes disposing opticallytransparent beads partially into an optically transparent layer, a beadpenetration depth into the optically transparent layer being selected tosubstantially maximize light transmission through the opticallytransparent beads, an absorbing layer overlying the opticallytransparent layer.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIGS. 1A and 1B illustrate a dispersing film having refracting beadswith exit apertures disposed within an absorbing layer;

FIGS. 1C-1E illustrate embodiments of a film according to the presentinvention;

FIGS. 2A and 2B respectively illustrate plots of light transmission vs.bead refractive index for the films of FIGS. 1A and 1B;

FIG. 3 illustrates a plot of light transmission vs. bead refractiveindex for the film of FIG. 1B for various values of cover layerrefractive index;

FIG. 4 illustrates a plot of light transmission vs. bead refractiveindex for the film of FIG. 1C for various values of cover layerrefractive index;

FIG. 5 illustrates an hexagonally close packed array of beads on a filmaccording to an embodiment of the present invention;

FIG. 6 illustrates a second embodiment of a film according to thepresent invention having a polarizing layer on a viewing side of thefilm;

FIG. 7 illustrates a third embodiment of a film according to the presentinvention having polarizing and quarter-wave retardation layers;

FIG. 8 illustrates a fourth embodiment of a film according to thepresent invention, having a Fresnel lens on an input side of the film;

FIG. 9 illustrates a fifth embodiment of a film according to the presentinvention, having a scattering layer above an absorbing layer;

FIG. 10 illustrates a sixth embodiment of a film according to thepresent invention, having scattering particles dispersed within theabsorbing layer;

FIG. 11 illustrates a first method of manufacturing the dispersing film;

FIG. 12 illustrates a second method of manufacturing the dispersingfilm;

FIGS. 13A-13B illustrate a third method for manufacturing the dispersingfilm;

FIGS. 14A-14C illustrate a fourth method for manufacturing thedispersing film;

FIG. 15 illustrates the refraction of light passing through a bead in atwo layer film having a clear top coating;

FIG. 16 illustrates a relationship between bead penetration depth andheating time for a thermoplastic transparent layer;

FIG. 17 illustrates fabrication steps in another method formanufacturing the dispersing film;

FIG. 18 illustrates fabrication steps in another method formanufacturing the dispersing film; and

FIG. 19 illustrates film contrast and transmission plotted as functionsof bead penetration depth.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to light dispersing films, and isbelieved to be particularly suited to light dispersing films for usewith rear projection screens and monitors. One of the advantages of theinvention is that the interdependence of the optical properties of thevarious components of the film is reduced, thus permitting a selectedfilm characteristic to be optimized without adversely affecting theother characteristics. This may be better understood with reference toFIG. 1A which illustrates, for comparison, a film 100, having asubstrate layer 102 covered by an opaque layer 104. In this film therefracting beads 106, 108 and 110 are supported in the opaque layer 104.Image light 114 is incident on the back side of the film 100 from asource. Some of the image light 114 is refracted on passing into thebeads 106, 108 and 110, and is directed to respective exit portions 116,118, and 120, where the opaque layer 104 is thinnest. Generally lightescapes through the exit portions 116, 118 and 120, into the substrate102 and out to the viewing side of the film 113.

The opaque layer 104 performs a number of functions, includingsupporting the beads, defining the exit aperture of the beads,controlling light passing through the interstices between beads andcontrolling ambient light. Ideally, the opacity of the opaque layer ishigh in order to control interstitial light and ambient light. However,the opacity of the opaque layer should be low for controlling theaperture of the bead. The reason for this is that the majority of thelight passing through the exit portion of a bead passes through aportion of the opaque layer 104. In order to permit a reasonable amountof light through the exit portion, and thus increase the transmission ofthe film, the opaque layer should have a low opacity. Therefore, acompromise is required in the optical properties of the opaque layer,and it is difficult, if not impossible, to optimize ambient andinterstitial light control at the same time as optimizing thetransmission of the film.

Another reason for low transmission through the film 100 arises becausebeads available for manufacturing the film 100 typically vary in size.For example, beads used in fabricating the film 100 may have an averagediameter of 60 μm, with a standard deviation of 12 μm. Consequently,when the beads are pressed into the opaque layer 104 during themanufacturing process, the larger beads, such as beads 106 and 110, arepressed down close to the substrate layer 102, while the smaller beads,for example 108, are not pressed as far. This results in a substantialthickness, d, of opaque layer 104 underlying the smaller bead 108, whichabsorbs the light exiting from the smaller bead 108.

An overlayer 112 covering the beads 106, 108 and 110 and the opaquelayer 104, as illustrated in FIG. 1B, may be used to protect the beads,support the beads in the film, or provide a flat surface for receivingother films.

The problems associated with the opaque layer 104 have a serious effecton the transmission of the film 100. This is illustrated in FIG. 2A,which shows the calculated variation of diffused light transmissionthrough the film 100 as a function of Δn for different values of d,where Δn is the change in refractive index experienced by the light onentering the beads. When there is no overlayer 112, Δn=n₁−1, where n₁ isthe refractive index of the bead. Where the overlayer has a refractiveindex of n₀, Δn=n₁−n₀. The values of d illustrated on the graph aregiven in microns. The absorption coefficient of the opaque layer wasassumed to be 0.5 μm⁻¹. The plot illustrates a maximum efficiency ofapproximately 75% for a glass bead refractive index of 1.5 for theimpractical case where the thickness, d, is negligible. Morerealistically, the thickness, d, is of the order of 1 μm or more,especially where there is a large variation in bead size. Thetransmission is significantly less when realistic values of d areconsidered, having a maximum of only about 42% when d=1 μm. Thetransmission drops further to about 16% for d=3 μm.

The refractive effect of the beads 106, 108 and 110 depends on thechange in refractive index experienced by light on passing into thebeads 106, 108 and 110. Since the refractive index of the cover layer112 is typically higher than that of air, the focusing power of thebeads 106, 108 and 110 is reduced in the presence of the cover layer112. Consequently, the light exiting the beads 106, 108 and 110 is lessconcentrated at the output portions 116, 118 and 120, with the resultthat a greater fraction of light passes through a significant thicknessof the opaque layer 104 and is absorbed. Therefore, the introduction ofthe cover layer 112 can affect the total amount of light transmitted bythe film 100.

This effect on the transmission is illustrated in FIG. 3, which showsplots of the light transmission through a beaded film calculated as afunction of bead refractive index for two values of refractive index,n₀, of the overlayer 112. In the calculations to produce the illustratedresults, it was assumed that d=0 μm and that the beads were packed atmaximum packing density. These plots illustrate that as the bead indexincreases, i.e. as Δn increases, the focusing effect of the beadincreases, thus constricting more light to the central, low loss portionof the bead output, and so overall transmission increases. Also, for agiven bead index, the transmission is higher for a lower cover layerrefractive index. However, few materials having a refractive index aslow as 1.3 are available for use in the cover layer, while severalmaterials are available that have a refractive index close to 1.5.Therefore, practical films of the sort illustrated in FIG. 1B sufferfrom low transmission.

There is, therefore, a substantial deficiency in the transmission ofbeaded dispersion screens of the sort just described. This transmissiondeficiency may be reduced by using mono-dispersed beads, i.e. beads ofuniform size. However, it is more expensive to produce beads havinglittle or no size variation, so the costs of the film are increased. Inaddition, practice shows that it is very difficult to reduce the valueof d to the sub-micron level where the affect on the transmission isnegligible, even when using mono-dispersed beads. Moreover, the use ofmono-dispersed beads does not solve the problems of reduced transmissionassociated with the cover layer.

A related problem arises from the fact that the opaque layer 104performs multiple functions. The opaque layer 104 absorbs interstitiallight passing between the beads, absorbs ambient light incident on thefilm 100 from the viewing side and also controls the diameter of theexit portion of each bead. Most of the light exiting from the beadspasses through a portion of the opaque layer 104 because the opaquelayer underlies substantially all of each bead. In order to increase thetransmission through the film 100, the light that exits the beads shouldnot be strongly absorbed, and so the absorption length of the opaquelayer should be long. On the other hand, in order to control ambientlight and interstitial light, the absorption length of the opaque layershould be short. These requirements on the absorption depth are inconflict. Thus, because the opaque layer 104 performs multiplefunctions, its optical properties may not permit optimal filmperformance.

In view of these problems with this film, another approach is needed.

The film of the present invention reduces the problems of lowtransmission without requiring the use of mono-dispersed beads. Instead,the exit face of the bead is not contained within the absorbing layer,but is positioned within an optically transmitting material. In oneparticular approach, an optically clear layer is disposed between theopaque layer and the substrate layer. The beads penetrate into the clearlayer, and so the opaque material between the exit portions of the beadsand the substrate is either minimized or avoided. In addition, it is theclear layer which controls the size of the exit aperture of the beads,and not the opaque layer. In another particular approach, the beadspenetrate through the absorbing layer into a clear substrate layer, andso the opaque material between the bead and the substrate is avoided. Inaddition, it is the penetration depth of the beads into the transparentsubstrate layer that controls the size of the exit aperture of thebeads, and not the opaque layer.

Thus, functions which impose conflicting requirements on the opaquelayer in the film described in FIG. 1A are decoupled, and it is possibleto increase the performance of the film beyond that possible with a oldfilm.

One particular embodiment of the invention is illustrated in FIG. 1C.The film 150 has a transparent substrate layer 152, on top of which isdisposed a layer of optically transparent material 153, having athickness, t. Typically, t is less than the radius of the bead, and mayhave a thickness of the order of one micron or so. The substrate layer152 may be formed from any suitable transparent material, includingacrylic. Where the film 150 is to be flexible, the substrate layer 152may be formed from a polyester or, if an optically isotropic material isrequired, from polycarbonate. The transparent material 153 may be formedby coating an optically clear polymer layer on the substrate layer 152.The polymer may be a compliant material, such as a thermo-plastic or apressure sensitive adhesive, or may be a curable clear layer, forexample a polymer that is thermally curable or curable by exposure toradiation.

An absorbing layer 154 overlies the layer of transparent material 153.The absorbing layer 154 typically has an optical density greater than 4so that the film contrast is kept high by extinguishing interstitiallight 165 passing between the beads 168 and 170. The absorbing layer 154may be formed from a polymer having a light absorbing agent such as adye, pigment, or carbon black disposed therein. The absorbing layer 154may also be a powder coating of carbon black, black dyes, opaqueparticles or inorganic particles, or such particles dispersed within abinding material. In one particular embodiment, the absorbing layer 154is formed from a clear binder having black particles disposedtherethrough. The binder may be, for example an acrylate or otherUV-curable polymer.

The absorbing layer may be made with a very high optical density. Also,unlike the single layer film 100, the absorbing layer 154 of the film150 need not be used to support the beads. Consequently, the absorbinglayer 154 may be made very thin relative to the bead diameter.

Beads 156, 158 and 160 are positioned to penetrate through the absorbinglayer 154 and into the transparent material 153 so that their exitportions 166, 168 and 170 lie within the transparent material 153. Thebeads may be glass beads, but may also be made from other opticallytransparent material including polymeric materials, such aspolymethylmethacrylate.

The layer of transparent material 153 may be made to be substantiallyless than the radius of the beads, even less than about 10% of the beadradius, so that there is sufficient absorbing material around the lowerportion of the bead to maintain high film contrast.

The film 150 may be used either with or without a cover layer 162covering the beads 156, 158 and 160, and the absorbing layer 154. Thecover layer 162 may be formed from any suitable transparent material,such as a polymer, sol-gel coating and the like. Advantages provided bythe cover layer 162 include protecting the beads, supporting the beadsin the film, and an improved diffuse light transmission. The cover layer162 may provide a flat outer surface upon which additional layers can bedisposed. The cover layer 162 a may conform to the surface topology ofthe beads, as shown in FIG. 1D, with the effect that the radius ofcurvature of the beads is reduced.

The exit portions 166, 168 and 170 of the beads 156, 158 and 160 arelocated within the transparent material 153. An advantage provided bythis embodiment is that, even though bead 158 is smaller than theadjacent beads 156 and 160, light exiting from the exit portion 168propagates into the transparent material 153, and through the substratelayer 152 to the viewing side of the film. Thus, the placement ofabsorbing material at the output of the bead is avoided. Thetransmission aperture in the film 150 is effectively controlled by thethickness of the transparent material 153. Hence, the transmission ofthe film 150 is higher than the transmission of the single layer film100.

Another embodiment of the film 180 is illustrated in FIG. 1E, where thebeads 156, 158 and 160 penetrate into the transparent layer 152, whichmay also act as a support for the film 180. The beads 156, 158 and 160penetrate into the transparent layer 152 to a penetration depth of aboutt. This embodiment is different from that shown in FIG. 1C in that thereis no separate layer of transparent material 153. In this film 180, thefunction of the transparent material 153 is also provided by thetransparent layer 152. This embodiment has a simpler construction thanthat of FIG. 1C because there are fewer layers. The transmissionaperture of this film 180 is effectively controlled by the penetrationdepth of the beads into the transparent layer 152. Hence, thetransmission of the film 180 is higher than the transmission of thesingle layer film 100.

Most of the additional embodiments presented below are described interms of having a substrate layer and a thin transparent layer intowhich the beads penetrate. It will be appreciated that this is notintended to limit the invention in any way, and the invention is alsointended to cover embodiments where the beads penetrate through theabsorbing layer into a single transparent layer, for example asillustrated in FIG. 1E.

FIG. 2B illustrates the calculated light transmission through the film150 as a function of refractive index change on entering the bead, Δn.It is assumed that the absorbing layer 154 has a thickness of 20 μm andan optical density of 0.5 μm⁻¹. A number of curves are shown,corresponding to different thicknesses of transparent material (orpenetration depth), t. The addition of the layer of transparent material153 (or the penetration of the beads into the transparent layer) reducesthe absorption of light transmitted by the beads, resulting intransmission values significantly higher than are obtained with thesingle layer film 100, as shown in FIG. 2A.

A further advantage of the films 150 and 180 is that, unlike the film100, the addition of a cover layer 162 does not introduce a severereduction in overall light transmission and, in fact, may even improvethe transmission. FIG. 4 illustrates the fraction of light transmittedby the film 150 or film 180 as a function of bead index, and for twovalues of cover layer refractive index. With the exception of thetransparent layer, the assumptions made in the calculations to producethese results were the same as used in FIG. 3. There are severalimportant differences between the results illustrated in FIGS. 3 and 4.First, it is important to note that the transmission of the film 150 orfilm 180 is substantially higher than the single layer film 100 over theentire range of bead refractive index illustrated. Second, for a givencover layer refractive index, the variation in transmission over therange of bead refractive index is less than for the single layer film100. These improvements in the transmission properties arise, at leastin part, because exit aperture of the beads is controlled by penetrationof the beads into the transparent layer 153 or substrate layer 152, andnot by the absorbing layer 154. Thus, the amount of light transmitted bythe beads is not significantly dependent on the refracting power of thebeads and so the introduction of the cover layer 162 has a smallereffect on transmission. Therefore, the refractive indices of the beadsand the cover layer can be independently selected to control the gainand viewing angle of the film 150 and 180, with little effect on thefilm's transmission.

A value of minimum thickness, of the transparent layer 153, t_(min),that corresponds to a film 150 having high light transmission is givenby:$t_{\min} = {r\left( {1 - {\frac{x^{2} + 8}{3x^{2}}\left\lbrack \frac{x^{2} - 1}{3} \right\rbrack}} \right)}$

where r is the radius of the bead and x is given by the ratio x=n₁/n₀,where n₁ is the refractive index of the bead and no is the refractiveindex of the cover layer 162. The transmission of light through the filmis reduced when the thickness of the transparent layer, t, is less thant_(min). For films having a thickness greater than t_(min), i.e. wheret>t_(min), the transmission is high. However, the film contrast reducesfor high values of t. Therefore, for applications where high filmtransmission and high contrast are important, the thickness of thetransparent layer is set at approximately equal to t_(min).

It will be appreciated that, where the beads penetrate through theabsorbing layer into a single transmitting layer, for example asillustrated in FIG. 1E, the above discussion is also applicable where tis taken to be the penetration depth of the beads into the substratelayer. In such a case, t_(min) is the penetration depth less than whichthe transmission of light through the film is non-optimal.

The expression for t_(min) given above holds for values of x less thanor equal to 2, for reasons which may be understood with reference toFIG. 15. The film includes a substrate layer 1502, a transparent layer1503, a light absorbing layer 1504 and a transparent cover layer 1512.An incoming light ray 1520 (solid line) is illustrated for the casewhere x is less than or equal to 2. The light ray 1520 enters the coverlayer 1512, having a refractive index of n₀, in a directionsubstantially normal to the film. The ray 1520 is refracted on enteringthe bead 1506, which has a refractive index n₁. The ray 1520 is againrefracted on exiting the bead 1506 into the transparent layer 1503 andthen substrate 1502; it is assumed that the transparent layer 1503 andthe substrate 1502 each have a refractive index given by n₂. On exitingthe film, the ray 1520 emerges at an angle η, as shown.

For x less than or equal to 2, the height between the substrate 1502 andthe point at which ray 1520 exits the bead 1506 defines the thickness,t(α), of the transparent layer 1503 through which the particular ray1520 travels. The angle of incidence, α, on the outer surface of thebead 1506 is greater for rays increasingly removed from the center-line,A, of the bead 1506. The thickness, t(α), increases with increasingincident angle, α, until it reaches a maximum, and then reduces as αapproaches 90°. The expression given above for t_(min) is an analyticalexpression that describes this maximum thickness.

On exiting the film, the ray 1520 emerges at an angle η. If, at someincident angle, α_(tir), η becomes larger than 90°, then the light istotally internally reflected at the lower surface of the substrate 1502.In other words, the angle ω becomes greater than the critical angle.This case is illustrated for ray 1522. The corresponding thickness atα_(tir) is defined as t_(tir). The value of thickness t_(tir) may bederived using numerical techniques. The minimum thickness for thetransparent layer 1503 which produces the highest transmission of lightis the larger of t_(min) and t_(tir). The solution for t_(min) does notexist for x less than or equal to 2. Therefore, for x less than or equalto 2, the minimum coating thickness for maximum light transmissionthrough the beads is given by t_(tir).

The effects of penetration depth on transmission through the film andfilm contrast are illustrated in FIG. 19. The transmission of the film(black dots) increases with increasing thickness, t, of the transparentlayer or, equivalently, with increasing penetration depth. The modelused to generate the results illustrated in FIG. 19 used the followingassumptions: the packing of the beads in the film is at maximum density(hexagonal close-packed) in a monolayer, all beads have a diameter of 60μm and penetrate to the same depth, t. In addition, the thickness, t,and the thickness of the absorbing layer sum to 30 μm and the absorptioncoefficient of the absorbing layer is 0.5 μm⁻¹. The substrate andtransparent layers are assumed to have refractive indices of 1.58, whilethe bead refractive index is 1.9 and the clear cover layer has arefractive index of 1.5.

The transmission for a value of t=0 is approximately 29%, rising toapproximately 70% for t=3.13 μm. The transmission remains atapproximately 70% for values of t higher than 3.13 μm. The “% BlackArea” is the percentage of the screen area, as seen by the viewer, thatis black. The contrast afforded by the film is proportional to thepercentage of black area and, therefore, a smaller fraction of blackarea represents a reduction in film contrast. The film contrast fallsfor transparent layer thickness increasing from zero to 10 μm.

Therefore, in order to obtain optimized film performance, the thicknessof the transparent layer, or, equivalently, the depth of penetration, ispreferably selected to maximize transmission through the film whilemaintaining as high a contrast as possible. Therefore, a preferredpenetration depth for the film whose properties form the basis for thecurves illustrated in FIG. 19, is approximately 3.1 μm, or a littleabove. A penetration depth significantly higher than about 3.1 μmresults in a reduced contrast without any offsetting benefit intransmission.

The beads may be disposed in the film 150 in a number of different arraypatterns. One such array pattern is a hexagonally packed pattern, whichadvantageously permits a high packing density, thus increasing theresolution and transmission of the film. A top view of a film 500 havingbeads arranged in a hexagonally packed pattern is illustrated in FIG. 5.The dotted line 502 shows a cross-section 6—6 through the film 500. Aportion of the section 6—6 is illustrated in FIG. 6 below.

FIG. 6 illustrates another embodiment of a film. This embodiment 600 issimilar to that illustrated in FIG. 1B, having a substrate layer 602, alayer of transparent material 603, an absorbing layer 604, beads, 606,608 and 610, and a transparent cover layer 612. Light 614 enters theilluminated surface 620 of the film 600, and exits through the viewingsurface 622. In addition, a polarizing layer 616 is positioned on theviewing side of the film 600. The polarizing layer 616 may be formedfrom absorbing polarizer film, which preferentially absorbs light havinga certain polarization, and which transmits light having the orthogonalpolarization. The polarizing layer 616 may alternatively be positionedbetween the layer of transparent material 603 and the substrate layer602.

An advantage provided by the polarizing layer 616 is that ambient light618 incident on the viewing surface 622 of the film 600 is generallyunpolarized and is therefore reduced in intensity by around 50% beforepassing into the film 600. This reduces the amount of ambient lightreflected out of the film, resulting in greater contrast in the imagetransmitted through the film 600.

It will be appreciated that efficient use of the image light 614requires the polarizing layer 616 to be oriented to permit hightransmission of the image light 614. For example, the image lightproduced by an LCD projector is linearly polarized, and should bealigned for preferential transmission through the polarizer 616. It willalso be appreciated that, where it is important to preserve thepolarization of image light passing through the film 600, the film 600is constructed from materials that do not alter the polarization oflight passing therethrough. For example, the materials used for thesubstrate layer 602, the layer of transparent material and the coverlayer 612 may be optically isotropic.

Another embodiment of a film 700 is illustrated in FIG. 7. The film 700includes a substrate layer 702, a layer of transparent material 703, anabsorbing layer 704, beads 706, 708, and 710, and a transparent coverlayer 712. In addition, a first polarizing layer 716 and a firstquarter-wave retarding layer 724 are positioned on the viewing side ofthe film 700, and a second quarter-wave retardation layer 726 ispositioned on the illuminated side of the film 700. A second polarizerlayer 728 may be provided on the second quarter-wave retardation layer726. The relative orientations of the polarizing layers 716 and 728 maybe chosen so that they operate as crossed polarizers.

An advantage of this arrangement is that it provides increaseddiscrimination against ambient light 718. The first polarizing layer 716absorbs around 50% of the incident ambient light 718, permitting alinearly polarized fraction to be transmitted. This linearly polarizedfraction of ambient light passes through the first quarter-waveretardation layer 724 to become circularly polarized. The circularlypolarized ambient light may be reflected from an interface between thecomponent parts of the film 700. Such reflected light passes once morethrough the first quarter-wave retardation layer 724 to become linearlypolarized in the direction of maximum absorption in the first polarizinglayer 716. Consequently, ambient light reflected within the film 700 isnot transmitted back out of the film, but is absorbed, thus reducing theamount of ambient light mixing with the image transmitted through thefilm 700. This reduction in ambient light advantageously enhances thecontrast of the film 700.

The film 700 may operate without the second polarizing layer 728,especially where the light 714 illuminating the film 700 is linearlypolarized.

Another embodiment of a film is illustrated in FIG. 8. The film 800includes a substrate layer 802, a layer of transparent material 803, anabsorbing layer 804, beads 806, 808, and 810, and a transparent coverlayer 812.

The lower surface 832 of the absorbing layer 804 lies on the layer oftransparent material 803 which, as stated above, is typically less thanhalfway up the bead. Alternatively stated, the lower surface 832 of theabsorbing layer 804 is closer to the substrate layer 802 than is a beaddiameter 836 that is substantially parallel to the substrate layer 802.The upper surface of the absorbing layer 804 may lie further from thesubstrate layer 802 than the diameter 836 which is substantiallyparallel to the substrate layer 802. When the absorbing layer 804 isrelatively thick, its absorption depth may be reduced by reducing theconcentration of the absorbing species. This may be advantageous wherethe absorbing layer 804 is also used as a scattering layer, as discussedbelow.

The film 800 may be provided with a Fresnel lens 830, or otherdiffractive optical component. An advantage provided by the Fresnel lens830 is that it may be used to collimate the image light 814 incident onthe film 800, or otherwise refocus the image light 814, when the imagelight 814 does not illuminate the film at a normal angle of incidence.For example, the Fresnel lens 830 may collimate light transmitted from apoint source, or a series of point sources.

Another embodiment of a film 900 is illustrated in FIG. 9. The film 900includes a substrate layer 902, a layer of transparent material 903, anabsorbing layer 904, beads 906, 908, and 910, and a transparent coverlayer 912. The film 900 also incorporates a scattering layer 931interposed between the cover layer 912 and the absorbing layer 904. Thescattering layer 931 may be formed from the same material as thetransparent material 903, with scattering particles 932 disposedtherein. The scattering particles 932 may be formed, for example, fromsmall particles of calcium carbonate, or any other suitable scatteringmaterial. The scattering particles may be formed from organic orinorganic material, and may be irregularly shaped or regularly shaped.

The scattering layer 931 may be used to increase the transmission oflight through the film 900. For example, without the scattering layer931, interstitial light beam 934 would normally be absorbed in theabsorbing layer 903. However, interstitial light beam 934 is scatteredby a scattering particle 932 into the bead, so that the light beam 936passes through the exit portion 920 and it transmitted through theviewing surface 922 of the film 900. The addition of a scattering layerprovides another parameter useful for controlling the gain and viewingangle of the film 900.

Another embodiment of a film 1000 is illustrated in FIG. 10. The film1000 includes a substrate layer 1002, a layer of transparent material1003, an absorbing layer 1004, beads 1006, 1008, and 1010, and atransparent cover layer 1012. In this embodiment, scattering particles1032 are disposed within the absorbing layer 1004. Light rays 1014 aretypically refracted by the beads 1006, 1008 and 1010, whereas theinterstitial light beam 1034 lies outside bead 1010 and would normallybe absorbed in the absorbing layer 1004. However, a scattering particle1032 scatters light beam into the bead 1010. The scattered light beam istransmitted by the film 1000 as beam 1036. Thus, the presence ofscattering particles in the absorbing layer 1004 may increase the amountof light transmitted by the film 1000. Additionally, the introduction ofscattering particles 1032 into the absorbing layer 1004 may alsoincrease the gain and viewing angle of the film 1000.

If the absorption depth of the absorbing layer 1004 is short, then onlylight scattered near the top of the absorbing layer, and close to thebead surface, will reach the bead, and light that has too long a pathlength within the absorbing layer will simply be absorbed, rather thanscattered. Thus, when the absorbing layer 1004 is used with scatteringparticles 1032, the absorption depth of the absorbing layer 1004 isadjusted so that a significant amount of light is scattered, rather thansimply absorbed.

The layer of transparent material 1003 may also be provided withscattering particles 1038 to scatter light 1040 transmitted by thebeads, thereby providing additional control over the gain and theviewing angle of the film 1000.

It will be appreciated that the size and number of scattering particlesin both the absorbing layer 1004 and in the layer of transparentmaterial 1003 may be adjusted to control the gain and the viewing angle.

FIG. 11 illustrates part of a first method of manufacturing a lightdispersing film 1100. First, a layer of transparent material 1103 islaid on the substrate layer 1102, and then an absorbing layer 1104 islaid over the layer of transparent material 1103. The beads 1106, 1108and 1110 are pressed through the absorbing layer 1104, and into thetransparent material 1103. The beads may be pressed into the absorbinglayer 1104 and the transparent material 1103 using, for example, a flatplate or a roller.

The transparent material 1103 may be an adhesive, such as a pressuresensitive adhesive, or a thermoplastic polymer such as polyethylene.

The absorbing layer 1104 may be softer than the layer of transparentmaterial 1103, for example, because of the intrinsic properties of theabsorbing and transparent materials. The absorbing material may alsohave a lower glass temperature, T_(ga), than the glass temperature ofthe transparent material, T_(gt), and the structure of layers 1102, 1103and 1104 may be heated to a temperature above T_(ga) to achieve thedesired softness of the absorbing layer 1104. The difference in softnessbetween layers permits the beads, as they are pressed towards thesubstrate layer 1102, to push the absorbing layer 1104 out of the way,so that little or no absorbing material remains on the lower surface ofthe bead at its exit portion within the transparent material 1103 toreduce the amount of light transmitted by the bead.

The absorbing layer 1104 may be additionally be formed of a materialthat does not readily wet the surface of the beads, so that thepossibility of absorbing material remaining in the bead's exit portionis reduced.

The transparent material 1103 may be an adhesive material, so that thebeads adhere to the transparent material 1103 as they are pushed intoit. The transparent material 1103 may also be curable, so that, once thefilm 1100 is cured, the beads are held, at least in part, by thetransparent layer 1103.

Once the beads have been pressed into the transparent material 1103, atransparent cover layer (not illustrated in FIG. 11) may be applied overthe top of the beads 1106, 1108 and 1110, and the absorbing layer. Ascattering layer may also be provided between the absorbing layer andthe cover layer. The cover layer provides protection for the beads andmay also be used to support the beads within the film. Other layers maybe applied to the film 1100, such as polarizing layers and quarter-waveretardation layers, scattering layers, Fresnel lenses and otherdiffracting optics, to produce the structures described hereinabove.

It is possible that portions of absorbing material may be present on theexit surfaces of the beads after the beads have been pushed through theabsorbing layer and into the transparent layer. If this occurs, thelight transmission through the beads may be reduced because the portionsof absorbing material occlude the output from the beads. This situationis illustrated in FIG. 13A, where the beads 1306 have been pushedthrough the absorbing layer 1304 into a transparent layer 1303 over asubstrate 1302. Some portions of absorbing material 1308 are present atthe output surfaces of the beads 1306. In one embodiment of a method forremoving the portions of absorbing material 1308, the substrate 1302 isremoved or etched away, leaving the transparent layer 1303 exposed. Thetransparent layer 1303 may then be etched, to remove both thetransparent layer 1303 and the portions of absorbing material 1308, toproduce the structure illustrated in FIG. 13B. Etching may be performedby an oxygen plasma etch or, where the transparent layer 1303 is formedof a photoresist-type material, etchable by an alkaline solution, theetching may be performed using a mildly basic water etch, such as0.01%-20% NaOH solution. The transmission of the beads 1306 is increasedby removal of the portions of absorbing material 1308. A replacementsubstrate layer 1320 may then be disposed over the exposed lightabsorbing layer 1304, to produce the structure illustrated in FIG. 13C.In addition, the replacement substrate layer 1320 may be formed from asingle layer of substrate material, as illustrated, or from one or morelayers (not illustrated).

FIG. 12 illustrates stages in a second method of manufacturing a lightdispersing film 1200. First, a layer of transparent material 1203 islaid on the substrate layer 1202, and then the beads 1206, 1208, and1210 are pressed into the transparent material 1203. The beads may bepressed, for example, using a flat plate or a roller. The transparentmaterial 1203 may be an adhesive, or at least tacky, material so thatthe beads adhere as they are pushed into the transparent material 1203.The transparent material 1203 may also be curable, so that, once thefilm 1200 is cured, the beads are held, at least in part, by thetransparent layer 1203.

Once the beads have been applied to the transparent material 1203, theabsorbing material 1204 is applied as a layer on top of the transparentmaterial 1203, in the interstices between the beads. Suitable absorbingmaterials include dyes, carbon black, and organic or inorganic pigmentswith a wide variety of sizes and shapes. The material can be dispersedeither in a liquid or a solid binder system. The absorbing material 1204can be applied by conventional coating techniques or powder coating. Inone particular embodiment of a method of applying the absorbing material1204 using a liquid dispersion, the surface tension of the coatingsolution may be high enough so that the solution does not readily wetthe surface of the beads 1206, 1208 and 1210. Consequently, theabsorbing material 1204 rolls off the beads into the interstitialspaces, and does not remain on the upper surfaces of the beads where itmight adversely affect the amount of light entering the bead.

After the absorbing material 1204 has been applied, a cover layer (notillustrated) may then be applied, or a scattering layer, followed by acover layer. Other layers may be applied to the upper and lower surfacesof the film 1200. For example, quarter-wave retardation layers,polarizing layers, Fresnel lenses and other diffracting optical layersmay be applied to produce the structures described hereinabove.Additionally, quarter-wave retardation and polarizing layers may beapplied between the substrate layer 1202 and the transparent material1203.

Another embodiment of a method of applying a layer of absorbing materialis illustrated in FIGS. 14A-14C. Beads 1406 having selected refractiveindex and size are first embedded in a transparent material 1403overlying a substrate 1402. A light absorbing layer 1404 is then coatedover the beads 1406. Some, or all of the beads 1406 may be completelycovered by the absorbing layer 1404, as illustrated in FIG. 14A. Theabsorbing layer 1404 is then etched back to reveal the upper surface ofthe beads 1406, illustrated in FIG. 14B. The extent of the etch isdetermined by the thickness of the light absorbing layer 1404 remainingover the beads 1406 after the absorbing layer 1404 is applied. Afteretching, the light absorbing layer 1404 remains sufficiently thick toeffectively absorb light incident thereon passing therethrough, asdiscussed hereinabove. After etching back the absorbing layer 1404, acover layer 1412 is applied, to produce the structure illustrated inFIG. 14C.

In another embodiment of a method of fabricating a film of the presentinvention, beads having a selected refractive index are disposed on apartially dried light absorbing layer coated on to a clear thermoplasticsubstrate. The film is then heated in an oven, for example a convectionoven, to a temperature sufficient to soften or melt the thermoplasticsubstrate. The softening or melting of the substrate layer enables thebeads to “sink” into the substrate layer. After the film is removed fromthe oven and cooled, the beads may be overcoated with a polymericcoating to control the effective refractive index of the beads, asdescribed above. This method may be used for fabricating a film wherethere is a single transparent layer below the absorbing layer.

A study of the “sink depth” of glass beads into a polymeric layer wasconducted, the results of which are illustrated in FIG. 16. In thisstudy, glass beads were dispersed on a polyethylene substrate and leftin an oven for a certain period of time. After removal from the oven,the beads were removed and the indentation depth was measured using aconfocal microscope. The results presented in FIG. 16 were produced attwo different temperatures, 100° C. (curve 1602) and 120° C. (curve1604). IT was found that the sink depth was greater for the highertemperature, and that the sink depth peaked at a heating time ofapproximately 5-7 mins.

As an example of this method, illustrated in FIGS. 17A-17B. A 3 milthick layer of polyethylene 1702 was coated with a solution containingcarbon black, polymeric binder and a cross-linker to form an absorbinglayer 1704, to produce the structure shown in FIG. 17A. The coating stepwas carried out using a knife-bar, but may be carried out using othermethods. The coating was partially dried in an oven at 60° C. for 45seconds. Glass beads 1706, having a refractive index of 1.9, weredispersed on the partially dried coating, to produce the structure shownin FIG. 17B. The sample was then heated in the oven at 130° C. for 3-5min. to allow the beads 1706 to penetrate into the substrate layer 1702,to produce the structure shown in FIG. 17C. The heating also resulted inthe evaporation of the solvent from the black coating and incross-linking the polymeric binder. The film 1700 was then coated with apolymeric layer 1712 having a refractive index of 1.52, to produce thestructure shown in FIG. 17D.

In another embodiment of a fabrication method, illustrated in FIGS.18A-18D, beads 1806 were made to penetrate into a transparent layer 1802of a thermoplastic polymer by heating the polymeric layer. A thinmetallic layer 1804 (200 nm thick copper) was vapor coated onto thebeads 1806 and transparent layer 1802, to produce the structureillustrated in FIG. 18A. The illustration is not drawn to scale. A layer1811 of Shipley type 1818 photoresist was then coated over the structurewith a knife coater. The photoresist layer 1811 was etched back using anoxygen plasma etch remove the photoresist from the upper surfaces of thebeads 1806, as illustrated in FIG. 18B. The film was then dipped in aferric chloride solution to etch the copper 1804 from the bead tops, toproduce the structure illustrated in FIG. 18C. The photoresist 1811protected the areas between the beads 1806 from etching. Finally, theremaining photoresist 1811 was removed by rinsing in water, resulting inthe structure shown in FIG. 18D, having beads 1806 embedded in atransparent layer, and with a layer of cooper 1804 operating as anabsorbing layer over the transparent layer 1802 and the lower portionsof the beads.

While various examples were provided above, the present invention is notlimited to the specifics of the illustrated embodiments. For example, acombination of scattering layers may be employed in a film, including ascattering absorbing layer employed in conjunction with a scatteringlayer above the absorbing layer.

It will be appreciated that the outer surfaces of the film may betreated with additional coatings for protection against physical damage,such as hard coatings and anti-smudge coatings. In addition,antireflection coatings may be provided on the outer surfaces to reducereflective losses.

In the above description, the positioning of layers has sometimes beendescribed in terms of“upper” and “lower”, “over” and “under”, and “top”and “bottom”. These terms have been used merely to simplify thedescription of the relative positions of different films illustrated inthe figures, and should not be understood to place any limitations onthe useful orientation of the film.

As noted above, the present invention is applicable to display systemsas a light dispersing film. It is believed to be particularly useful inback projection displays and screens. Accordingly, the present inventionshould not be considered limited to the particular examples describedabove, but rather should be understood to cover all aspects of theinvention as fairly set out in the attached claims. Variousmodifications, equivalent processes, as well as numerous structures towhich the present invention may be applicable will be readily apparentto those of skill in the art to which the present invention is directedupon review of the present specification. The claims are intended tocover such modifications and devices.

We claim:
 1. A filter for transmitting light with a selected angulardispersion, comprising: a layer of optically transparent material havinga support surface; a plurality of beads of optically transmittingmaterial having a selected shape and refractive index and arranged in asingle-layer array on the support surface, each bead at least partiallydisposed within the layer of optically transparent material to producean interface therewith, the interface defining a clear exit aperture;and a layer of light absorbing material having a selected thickness andbeing affixed to the layer of optically transparent material, forcontrolling ambient light rejection of the light filter and to reducelight transmission through interstices formed by the plurality of beads;wherein penetration depth of at least one of the beads into the layer ofoptically transparent material is selected to substantially maximizetransmission of light through the filter while maintaining film contrastfor the maximum light transmission.
 2. A filter as recited in claim 1,wherein the layer of optically transparent material includes apenetration layer on a substrate layer, and the beads penetrate into thepenetration layer.
 3. A filter as recited in claim 1, further comprisinga layer of light transmitting material deposited over the layer of lightabsorbing material and the plurality of beads, the layer of lighttransmitting material having a refractive index that operates with therefractive index of the beads, to provide the light filter with theselected angular dispersion of transmitted light.
 4. A filter as recitedin claim 3, wherein the layer of light transmitting material secures theplurality of beads partially disposed within the layer of opticallytransparent material.
 5. A filter as recited in claim 3, wherein thelayer of light absorbing material covers a substantial portion of alower half of each bead.
 6. A light dispersing film, comprising: anoptically transparent layer; optically transmitting beads arranged topenetrate at least partially into a first surface of the transparentlayer to a preselected penetration depth, the beads defining exitapertures at interfaces between the beads and the transparent layer; andan absorbing layer disposed on the transparent layer, in intersticesbetween the beads; wherein the penetration depth is selected tosubstantially achieve maximum transmission of light through the filmwhile maintaining essentially optimum contrast for the maximum lighttransmission.
 7. A film as recited in claim 6, wherein the opticallytransparent layer includes a penetration layer over a substrate layer,and the beads are arranged to penetrate the penetration layer, thepenetration layer having a predetermined thickness less than a beadradius.
 8. A film as recited in claim 6, wherein the penetration depth,t, is approximately equal to${t = {r\left( {1 - {\frac{x^{2} + 8}{3x^{2}}\left\lbrack \frac{x^{2} - 1}{3} \right\rbrack}} \right)}},$

where r is a bead radius and x is a ratio given by a bead refractiveindex divided by a transparent cover layer refractive index, and x isless than or equal to
 2. 9. A film as recited in claim 6, furthercomprising a transparent cover layer disposed over the absorbing layerand the optically transmitting beads.
 10. A film as recited in claim 9,wherein the transparent cover layer is adapted to secure the beadspenetrating into the optically transparent layer.
 11. A film as recitedin claim 9, further comprising a first quarter-wave retardation layerdisposed over the transparent cover layer, a second quarter-waveretardation layer disposed below the optically transparent layer, and afirst polarizer layer disposed below the second quarter-wave retardationlayer.
 12. A film as recited in claim 11, further comprising a secondpolarizer disposed over the first quarter-wave retardation layer.
 13. Afilm as recited in claim 9, further comprising a first quarter-waveretardation layer disposed over the transparent cover layer and apolarizing layer disposed over the first quarter-wave retardation layer.14. A film as recited in claim 9, further comprising a Fresnel lensdisposed over the transparent cover layer.
 15. A film as recited inclaim 6, further comprising a polarizing layer disposed over a secondsurface of the transparent layer.
 16. A film as recited in claim 15,further comprising a second quarter-wave retardation layer disposedbetween the polarizing layer and the second surface of the transparentlayer.
 17. A film as recited in claim 6, wherein at least one of thetransparent layer and absorbing layer include light scatteringparticles.
 18. A film as recited in claim 6, further comprising ascattering layer disposed over the absorbing layer, and a transparentcover layer disposed over the scattering layer and the opticallytransmitting beads.
 19. A film as recited in claim 6, wherein thetransparent layer is a curable adhesive layer and the opticallytransmitting beads adhere to the adhesive transparent layer.
 20. A filmas recited in claim 6, wherein the transparent layer is selected from apressure sensitive adhesive and a hot melt adhesive, and the opticallytransmitting beads are pressed into the transparent layer.
 21. A film asrecited in claim 6, wherein the absorbing layer is a metallic layer. 22.A film as recited in claim 6, wherein the absorbing layer extends fromthe transparent layer below a bead diameter substantially parallel tothe substrate to a position on the bead surface above the bead diameter.23. A film as recited in claim 6, wherein the absorbing layer isdisposed within interstices between beads and has an upper extent lyingcloser to the transparent layer than a bead diameter substantiallyparallel to the substrate.
 24. A film as recited in claim 6, wherein theoptically transmitting beads are formed from one of glass and anoptically transparent polymeric material.
 25. A light dispersing film,comprising: optical absorbing means for preventing transmission of lighttherethrough; first optical refracting means for refracting lightpassing therethrough, the first optical refracting means disposedthrough the optical absorbing means so as to permit light entering theoptical refracting means to exit the optical refracting means; andoptically transmitting support means for supporting the opticalabsorbing means, the first optical refracting means being disposed topenetrate into the optically transmitting support means to a penetrationdepth selected to essentially maximize light transmission through thefilm.
 26. A light dispersing film as recited in claim 25, furthercomprising a second optical refracting means disposed over the firstoptical refracting means for altering an effective refractive index ofthe first optical refractive means.
 27. A light dispersing film asrecited in claim 25, further comprising light scattering means disposedwithin the film for scattering light passing through the film.
 28. Alight dispersing film, comprising: an optically transparent layer;optically transmitting beads arranged to penetrate at least partiallyinto a first surface of the transparent layer to a preselectedpenetration depth, the beads defining exit apertures at interfacesbetween the beads and the transparent layer; an absorbing layer disposedon the transparent layer, in interstices between the beads; and atransparent cover layer disposed over the absorbing layer and theoptically transmitting beads.
 29. A film as recited in claim 28, whereinthe optically transparent layer includes a penetration layer over asubstrate layer, and the beads are arranged to penetrate the penetrationlayer, the penetration layer having a predetermined thickness less thana bead radius.
 30. A film as recited in claim 28, wherein thepenetration depth, t, is approximately equal to${t = {r\left( {1 - {\frac{x^{2} + 8}{3x^{2}}\left\lbrack \frac{x^{2} - 1}{3} \right\rbrack}} \right)}},$

where r is a bead radius and x is a ratio given by a bead refractiveindex divided by a transparent cover layer refractive index, and x isless than or equal to
 2. 31. A film as recited in claim 28, wherein thetransparent cover layer is adapted to secure the beads penetrating intothe optically transparent layer.
 32. A film as recited in claim 28,wherein at least one of the transparent layer and absorbing layerinclude light scattering particles.
 33. A film as recited in claim 28,further comprising a scattering layer disposed between the absorbinglayer and the transparent cover layer.
 34. A filter as recited in claim1, further comprising a substrate layer disposed on an output side ofthe optically transparent layer.
 35. A film as recited in claim 6,further comprising a substrate layer at an output side of the opticallytransparent layer.
 36. A film as recited in claim 35, wherein the beadspenetrate through the optically transparent layer to the substratelayer, and the optically transparent layer has a thickness less than abead radius.
 37. A film as recited in claim 28, further comprising asubstrate layer at an output side of the optically transparent layer.38. A film as recited in claim 37, wherein the beads penetrate throughthe optically transparent layer to the substrate layer, and theoptically transparent layer has a thickness less than a bead radius.